2008 Northern Fur Seal Pup Production and Adult Male Counts on the Pribilof Islands, Alaska
Pup production and adult male northern fur seal count
Numbers of northern fur seal, Callorhinus ursinus, pups were estimated using a mark-recapture method, shearsampling, on the Pribilof Islands during August 2008. We estimated 102,674 (SE = 1,084, Table 1) pups were born on St. Paul Island and 18,160 (SE = 288, Table 1) pups were born on St. George Island. To minimize disturbance, dead pup counts were only conducted at 4 sample rookeries on St. Paul Island and 3 sample rookeries on St. George Island. As in 2006, this change in the dead pup counting procedure resulted in a different protocol (ratio estimation) for the calculation of the estimate of pups born. The observed pup mortality rates of 5.3% on St. Paul Island and 5.4% on St. George Island were relatively low, yet higher than estimates obtained in 2004 and 2006. Pup production was estimated on Sea Lion Rock, a small island approximately 500 m from St. Paul Island, for the first time since 2002. The estimated number of pups born on Sea Lion Rock was 6,741 (SE = 80). Pup production on Sea Lion Rock decreased by 22.8% since the last estimate in 2002. The 2008 pup production estimate for St. Paul Island is 6.6% less than the estimate in 2006 (Figure 1). The 2008 pup production estimate for St. George Island is 6.4% greater than the estimate in 2006. The overall decrease in pup production for St. Paul and St. George Islands combined from 2006 to 2008 is approximately 4.9%. Since 1998 pup production on St. Paul Island has declined at an annual rate of 5.7% (SE = 0.38) and on the Pribilof Islands (St. Paul and St. George Islands) at an annual rate of 5.2% (SE = 0.40).
Adult male northern fur seals on St. Paul, and St. George Islands (Table 2) were counted over the period July 8 to 15, 2008. Counts of territorial males with females ("harem" males) on St. George increased by 8.2% compared to 2007. Idle males on St. George increased in comparison to 2007 by 14.1%. On St. Paul the idle males decreased by 4.2% and the "harem" males increased by 15.4% (Figure 2). Overall, the total number of adult males counted on the Pribilof Islands in 2008 increased by 4.6% from 2007 to 10,612. The total number of territorial males with females increased by 14.2% on the Pribilof Islands.